Defect or anomaly detection in structures is often important in determining maintenance intervals, or for determining whether structures require repair or replacement. Non-destructive detection of structural anomalies may be desired, and the ability to perform timely and effective examination of objects may not necessarily be made easier when the objects are large, may be remotely located relative to large population centers, and may be subject to harsh geographic or climatic conditions.
By way of example, the inspection of pipelines is a task of some interest and economic importance, in particular as it pertains to pipelines for carrying hydrocarbon gases and oils, although pipelines for transporting other fluids and slurries are also known. A typical pipeline for carrying gas, oil or water may run for many miles between pumping stations. The pipeline may be exposed to the weather. That weather may include a corrosive atmosphere, be it a salt spray environment or some other. The pipeline may run through regions of greater or lesser humidity. There may be extremes of heat and cold. In some places the pipeline may be carried above ground on spaced supports. In others it may be buried, or partially buried. In locations in which the pipeline is buried, the surrounding stratum may have a high or low moisture content, and may be alkaline or acidic. The fluid, or slurry to be carried in the pipeline may itself not be benign, but may be of an aggressive nature, and may be abrasive or corrosive, or both. The material flowing in the pipeline may be under significant pressure, perhaps in the thousands of psi., and may be at an elevated temperature, possibly in the range of 80-100 C. This environment may effect not only the life of the pipeline and the nature of the defects that may be expected to be found in a section of pipe over time, but also the tools used for monitoring and maintaining the pipeline. Stress cracking and stress corrosion may occur or be hastened by movement related to temperature change, earthquakes or tremors, ground settling, vibration from fluid movement, and pressure changes in the medium during operation.
Pipelines are subject to many different kinds of defects. There may be internal or external corrosion. There may be fatigue cracks, most typically externally initiating. There may be cracks or inclusions in the welded joints near flange connections. There may be dents or cracks caused by external factors. There my be an out of round, or ovality, condition. It may be that a defect in the pipewall of a pipeline may be relatively benign, and may not be life limiting. It may be of a size that may permit scheduled removal at the next convenient maintenance interval, rather than immediate removal on a more urgent, and costly, basis. Inasmuch as the removal and replacement of, for example, buried pipe in a remote location may not be overly convenient, knowledge of whether a pipe is at or near a certain defect size limit may be quite helpful.
It is known to monitor the condition of pipelines by passing monitoring tools down the pipe. Such tools tend generically to be known as pipeline “pigs”. A “pig” is somewhat of a plug, or slug, that fits within the pipe and has a generally squat shape—namely a relatively low length to diameter ratio—that may permit the pig to get around bends in the pipe. A pig may be a “dumb pig” or an “intelligent pig”. An intelligent pig usually has sensing and recording equipment. The general manner of operation is that the pig is inserted into the flow path, and then the flow of fluid carries the pig along the pipe. Usually the pig has a body, and the body has one or more seal rings or skirts (usually one upstream and one downstream) such as may tend to wipe along the pipewall, and for which the conventional terminology is a “cup”. The cups tend to be consumable polyurethane skirts that are replaced after each run through a pipeline section. It may be that more than one pig may be sent down the pipeline at the same time, with the pigs being hooked together in a train like manner at articulations. These articulations permit the train of pigs to pass through corners in the pipeline. One reason why more than one pig may be employed is that a second pig may carry the electrical power source (e.g., batteries) for the electrical equipment carried by the “intelligent” pig. When the pig is inserted, a pressure build-up behind the upstream seal (and a reduction below the downstream seal, as may be), causes the pig to be carried along, such that the motive power for pig operation, and the speed at which the pig moves, are dictated by the pumping power of the fluid driving pump. Typical fluid speeds may vary greatly, from perhaps as low as 0.5 m/s to about 10 m/s for a liquid, and perhaps 5 m/s to 50 m/s for a gas.
The measurement of defects in pipelines poses a number of challenges. First, it may be helpful to be able to differentiate between, for example, a build up of corrosion, and a fatigue crack, or between either of them and a dent. Second, an intelligent pig may have a large power requirement, it has to travel with a big power supply or it can only go a relatively short distance in the pipe before it must be removed, and the power supply replaced or recharged. Alternatively, the amount of data to be recorded my be too great, and periodic removal and downloading may be required. Further, where portions of the pig, such as brushes (e.g., electromagnetic brushes of feeler gauges), contact the pipewall during motion, or where sensors are carried in a relatively exposed manner, the maintenance required to overhaul the pig in preparation for its next run through the pipe may in itself be an expensive, laborious and time consuming task. The post-run signal processing may itself be quite an undertaking, and may not yield results for several days. An improvement in any one of these things would be welcome—be it a reduction in power consumption, real time signal processing that reduces the amount of data to be stored, a reduction in maintenance requirements, an improvement in the resolution of the size of defect that can be detected, or an improvement in the ability to discriminate between types of defects.